It has been known that some kinds of platinum complexes have antitumor actions; particularly, cisplatin [cisdiamminedichloroplatinum(II)] has been used clinically in treatment of malignancy, for example, disseminated testicular and ovarian carcinoma. Cisplatin, however, has serious side effects such as renal toxicity, myelosuppression, and ototoxicity. Recently, the occurence of malignancy and death rate owing thereto are on the increase, and it is desirable to develop antitumor agents of which the activity is strong but less toxic. In order to achieve the above purpose, chemical modification of platinum complexes has been attempted extensively. Representatives of the platinum complexes disclosed in literatures are malonato (1,2-diaminocyclohexane)platinum(II)[Japanese Unexamined Patent Publication No. 53-31648], sulfato (1,2-diaminocyclohexane)platinum(II)[Japanese Unexamined Patent Publication No. 54-44620], and the like.